The concept of a continuum of antiarrhythmic drug effect has been developed by the Principal Investigator's previously funded studies. Frequency-dependent excitability block occurs because drug block of upstroke velocity is greater after an action potential than before. Associated with each drug there is a unique half-time which governs the recovery of upstroke velocity of intracellularly recorded action potentials. Smaller drug structures, except for those having very low lipid solubility, manifest faster block escape rates (smaller half-times). Specific tests of the correctness and utility of this new drug continuum will be made, including: tests of newer antiarrhythmic drug structures for appropriate fit within the memory continuum, tests of the continuum as it applies to a model of depressed myocardium, and assessment for possibly different effects of drugs from different parts of the spectrum on automaticity, contractility and action potential duration. These proposed studies should improve our understanding of how some antiarrhythmic drug structures might selectively block fibrillatory but not normal rates of impulse propagation in the heart and, in addition, identify which drugs might block excitability most selectively and effectively in depressed myocardium.